Field of the invention
The present invention relates to a printed circuit board (PCB) and in particular to a printed circuit board comprising conductive layers separated by insulation layers of dielectric material, at least one conductive layer being patterned and having at least one signal line embedded in an insulation material.
Description of the Related Art
Increasing miniaturization and extreme electronic component density as well as the necessity to transfer large amounts of data at high speed, can create serious problems with respect to signal integrity in PCBs. A specific problem here is the desire to configure signal lines with predetermined impedance. In order to avoid signal losses due to reflexions at interfaces with other signal lines, it is necessary to adjust the line impedance already during the manufacture of a PCB as accurately as possible. Leakage currents, which should be as small as possible, constitute a further problem. A further problem, typically for high speed lines, is associated with return currents in high-frequency applications, leading to an uncontrolled impedance of the transmission line, build by a signal line and a ground plane. A further problem is given by crosstalking and electromagnetic noise, received and/or emitted by signal lines, since the current paths may be considered as an antenna, receiving and transmitting signal energy, thereby creating electromagnetic interference.
EP 1 443 811 A2 deals with some of these problems and discloses a multilayer PCB with signal lines suitable for systems operating at 800 MHz and more.
Due to the limited space it is not expedient to use all the recommended measures used in conventional circuit design for avoiding cross-talking and electromagnetic noise, e.g. widening of spaces between signal lines, which would lead to an undesirable increase of the thickness of the PCB as such.
FIG. 1 is a cross sectional view of a PCB having three conducting layers and three dielectric insulation layers. In this example of a conventional PCB 1 on a support core 2 there is arranged a first structured conductive layer 3 made of conductive material, mostly copper, including two signal lines 4, 5 followed by a first prepreg-layer 6, a second conductive layer 7, which may be structured and may contain a signal line 8, followed by a second prepreg-layer 9 and, arranged on said second prepreg layer 9, a third uppermost conductive layer 10 which may be structured in a well-known manner too. Crosstalking can occur between the two signal lines 4, 5 or between these signal lines 4, 5 and signal line 8 or other components of the PCB, not shown.
Further it should be noted that the impedance of the signal transmission line is a function, amongst others, of the distance between the signal lines 4, 5 and ground, defined primarily by the conductive layer 7, the width of the lines 4, 5 and the relative permittivity εr of dielectric layer 6. At a given width of a signal line higher impedance may be reached by using a dielectric layer with a low relative permittivity εr and/or by increasing the distance between a signal line 4, 5 and the conductive layer 7. Since the relative permittivity is determined in most cases by the commercially available materials, mostly prepregs, FR4, Polyimide etc., one have to increase the distance between the signal lines 4, 5 and the conductive layer 7, which leads to an undesirable increase of the thickness of the PCB as such. Further problems are cross-talking and interferences: areas where high speed signals are close to each other are the most critical areas to get noise or disturbance from the adjacent signals.
It is further to mention that in order to reduce loss of electrical signal in high-frequency applications the PCB must show low dielectric constant and low dielectric loss.